This invention relates to semiconductor wafer processing, and more particularly to a removable semiconductor wafer susceptor which can be used in batch processing of semiconductor substrates.
For common semiconductor films such as silicon nitride, polysilicon, and thermal oxides, substrate processing usually proceeds by elevating the substrate to some process temperature, conducting the process, and finally cooling the substrate. Generally, most processes are conducted in a 200 mm batch furnace where substrates (hereafter referred to as wafers) are placed in a vertically stacked arrangement. Because of process and throughput requirements, the wafer stack often undergoes rapid heating and cooling at the beginning and end of the process. However, some thermal ramping limits exist at higher processing temperatures. It is now known that for 300 mm wafer, serious limitations exist on wafer heating/cooling rates and maximum process temperatures, well below the operational limits of the processing equipment.
The gravitational force and elevated process temperature (typically above 850xc2x0 C.) cause considerable stress on the silica on wafer, leading to situations where slip and plastic deformation may occur. Fast thermal ramping can further degrade the situation because within-wafer (WinW) thermal gradients from uneven heating of wafers in a vertical stacked arrangement may cause slip to occur even before the process temperature is reached. Of course, fast thermal ramping is employed to increase productivity by decreasing the overall cycle time or reduce thermal budget by decreasing the ramping cycles. Therefore, a serious situation arises for high temperature processing of 300 mm substrate, especially in batch processing environments. Additionally, even if slip does not occur, the induced thermal gradient on the wafer may be of sufficient magnitude as to cause significant differences in the thermal histories of the die spread across the wafer. This will result in an unexpected die performance variation between the wafer center and edge locations.
Two approaches can be taken to solve this slip problem. One approach is to improve the wafer""s chemical and mechanical characteristics, such as decreasing the oxygen precipitate concentration within the silicon wafer. This approach is an area of responsibility for the wafer manufacturers. The other approach is to improve the substrate susceptor(support) design.
The current industry standard for vertical batch wafer processing is the ladder boat and its variations (FIG. 1). The wafer boat is a holding device shaped like a hollow cylinder and is made of, for example quartz which has high heat resistance and high chemical stability. It comprises, for example, four vertical support rods, each having grooves. The wafer boat holds wafers, each set at its circumferential portion in the four grooves made in the four support rods. Hence, the wafers are held parallel to one another and one above another. This is the simplest design for vertical batch processing. However, it does not provide the most optimum mechanical support possible with respect to gravitational forces. Also, the standard ladder boat provides little reduction in thermal gradients. The ladder boat""s greatest advantages are its low cost and compatibility with standard automation.
Two previously developed innovations have addressed the WinW wafer thermal issue for batch processing. The first wafer support method, shown in FIG. 2, was developed and patented by Tokyo Electron Ltd. (TEL). This xe2x80x9cringxe2x80x9d support method uses a ring of material (typically quartz) designed to come into physical contact with the edge of the wafer. The addition of mass near or at the wafer""s edge reduces the WinW thermal gradient because of the increase in heat capacity and change in radiation view factors. The method also provides a larger area of mechanical support than a ladder boat. The method gives good performance on 200 mm wafers, as thermal WinW gradients are controlled to under 10xc2x0 C. for fast thermal ramps (above 75xc2x0 C./min). However, this support method is complex and such designs are more expensive to manufacture and purchase. Additionally, this method requires more complex automation to load and unload wafers from the support appliance, leading to added cost for the associated support automation.
Another approach found in the prior art (previously patented by SVG, Thermco Systems) is the xe2x80x9cbandxe2x80x9d method as shown in FIG. 3. Here, a thin band of material, typically quartz, is placed around the edge of the wafer, but not in intimate contact. The quartz material is either opaque or mechanically modified to be translucent. This method, like the ring support, reduces or screens incident radiation onto the wafer""s edge, while permitting radiation through the unblocked areas and onto the wafer""s center. Although not as effective as the ring support method shown in FIG. 2, the xe2x80x9cbandxe2x80x9d method does reduce WinW thermal gradients and can be manufactured at a lower cost.
Other approaches to wafer support methodologies have been previously explored by others and are well known within the industry. In FIG. 4A, the best theoretical point contact support at a single radius value is shown. This method places point supports at 70% of the radial distance from the center to the wafer""s edge, to balance the weight of the wafer on either side of the support and reduce gravitational stress effects. This approach when implemented in a ladder boat configuration will provide better support, but the cost will be greater due to the additional manufacturing complexity of very long support tabs. Also, this method does not address the WinW thermal gradient problem. A corresponding analogy exists for the ring support (point contact) where the location for a single ring would be also at 70% of the radial distance from the center to the wafer""s edge. In this case, the ring support""s axial symmetry greatly improves the control of the gravitational stress magnitude and symmetry compared to the ladder boat method. FIG. 4C shows the absolute best theoretical support design possible, as all pints on the wafer are mechanically supported. Clean, simple, and efficient mechanical wafer loading and unloading for this design becomes a serious problem, if not impossible, with current automation technology.
The vast majority of single wafer processing equipment currently use supports shown in FIGS. 4B and 4C. Here either a ring of material or a flat plate or susceptor composed of quartz, SiC or similar material supports the wafer. These design are preferred for reasons of simplicity or reduction of thermal mass to permit rapid wafer heating and cooing (up to 100xc2x0 C./sec). The supports in FIGS. 4B and 4C are not necessarily employed for thermal WinW control in single wafer processing equipment because they rely on heating element design to accomplish WinW thermal uniformity. In some cases, there may be some benefit based on material selection with reducing thermal non-uniformity. As an added benefit, gravitational forces are reduced and in the case of FIG. 4C, are completely eliminated if the right support material is used. However, these designs do add complexity to the method of wafer handling and are best suited for single wafer environments where the automation comprises a larger percentage of the overall equipment set and cost.
WinW Thermal Gradients
The primary issue with batch processing and rapid heating of large substrates is the resultant thermal gradients, as demonstrated in FIG. 5. During the heating phase of the process cycle (see FIG. 5A), the edges of the wafer receive the majority of the incident radiation and as a result heat up at a faster rate. Heating of the interior regions of the wafer is chiefly accomplished by thermal conduction through the substrate itself. As a result, a xe2x80x9cbowlxe2x80x9d-shaped thermal profile forms across the wafer. This thermal gradient can add to the gravitational stress andxe2x80x94if large enoughxe2x80x94cause warping, bowing, plastic deformation, and slip to occur. A solution to this problem would be to increase the pitch of the wafer stack, thereby increasing the radiation view factor for the wafer center.
As in the case for heating, rapid cooling of the wafer (see FIG. 5B) can also have negative effects. Efficient radiative cooling of the wafer""s edge occurs because of a large exposed area (large angular exposure to the heater walls) at the wafer""s edge. The interior regions of the wafer have smaller exposed angular area to the outside and thus cool inefficiently through radiation. The central region of the wafer mainly cools through thermal conduction from the wafer center to the edge where then energy is more effectively radiated away. As a result, a xe2x80x9cdomexe2x80x9d-like thermal gradient is formed across the wafer. This thermal gradient can add to the gravitational stress andxe2x80x94if large enoughxe2x80x94cause warping, bowing, plastic deformation, and slip to occur. Like heating, a solution would be to increase the radiation view factor for the wafer center.
Given a particular support design, the magnitude of this WinW thermal gradient coupled with the process temperature determines whether slip conditions exist. FIG. 6 shows the difference in slip curves between a ladder-type and a ring boat. For a given WinW thermal gradient and process temperature, the wafer will tend to exhibit slip if the process condition lies on the right-hand side of the slip curve. FIG. 6 shows that the maximum allowable delta T decreases rapidly with increasing wafer edge temperature.
As seen in FIG. 7, the ladder boat (3 point support) would not be sufficient for processes requiring temperatures above 850xc2x0 C., as slip and possible plastic deformation would occur. Increasing the number of point supports for a ladder boat or decreasing the oxygen precipitate concentration would help. Increasing the number of point supports and relocating them to the optimum locations would shift the slip curve to the right and permit a larger allowable WinW thermal gradient the processing temperature. The disk and ring supports lie near the limit for such improvements. The ring boat (ring support) would be adequate for the high temperature processes, but the complexity of wafer automation would be a disadvantage.
The present invention relates to a removable substrate susceptor. In FIG. 8, the susceptor (xe2x80x9cDrop-Through Diskxe2x80x9d) provides a (substrate) support design that is generally flat and makes contact with the majority of the substrate (herein referred to as wafer) as to eliminate the gravitational stress component, such as a disk (FIG. 9), ring (FIGS. 10 and 11), or plate. A support is designed to be removable from the boat with industry automation technology. The desired support is composed of a material or composite with a higher thermal conductivity and stronger mechanical strength than Si, such as SiC, and be compatible with the chemistry and gas(es) present in the process chamber. The support is thin in cross section so that it adds minimal mass to the thermal load while thick enough to provide adequate mechanical support. The support""s exact thickness can be determined experimentally so that it eliminates slip at the desired processing conditions. Theoretical calculations with MacWafer(copyright) suggests that the thickness for a SiC support should be between 1 to 2 wafer thicknesses (xcx9c750-1500 microns). The wafer contact side of the support should be processed flat but have a rough texture to avoid stiction and wafer drift movements.
The wafer support itself is further supported by the support members (tabs/fingers) 33 of a larger substrate support assembly 16, such as a ladder boat. The design of the larger substrate support assembly 16 (hereafter called a boat) can be varied with the only requirement is that it has support members 33 extending long enough towards the wafer center to make contact and establish adequate mechanical support with he wafer support. For example, in FIG. 9, the assembly can resemble a conventional ladder boat but with longer fingers. The simplest boat design is to be a 3-rail type with 120 degree spacing between rails. This ensures the best weight distribution of the wafer/support combination onto the boat. To maintain ring alignment, a third recess 31 (small recesses) are cut into the support members 33 or small tilt on fingers (not shown) are designed for wafer support contact point at the xcx9c100 mm radial point.
Likewise, proper wafer and support alignment can be maintained while they are on the end-effector (wafer transfer mechanism) by designing the end-effector to have 2 different sized chamfered recesses or xe2x80x9ctrenchesxe2x80x9d (FIG. 12). When placed onto the end-effector, the support slides into the deepest (and smallest) second recess 19 while the wafer 12 sits on the upper, first recess 17.